Tandem solar cells that incorporate semiconductors having various energy bandgaps using one or more p-n homojunctions and/or heterojunctions are generally integrated via p+-n+ tunnel junctions. FIG. 1 illustrates a tandem solar cell structure and shows the integration of two (2) n-p homojunction cells via one (1) p+-n+ GaAs tunnel junction [1]. As shown, the tandem solar cell includes a top cell 12 and a bottom cell 13 separated by a tunnel cell 19, where the bottom cell is made of n-p GaAs and the top cell is realized using n-p GaInP (Eg=1.86 eV). As is known, tandem cells efficiently harness solar energy and reduce excess energy losses by absorbing different energy photons in semiconductor layers having different energy gaps. For example, a two junction cell using a GaAs substrate and operating at ˜25.7% efficiency, as shown herein, harnesses photon energies above 1.86 eV in the top cell 102 and photon energies in the range of 1.86 eV-1.42 eV in the bottom cell 104. In some instances, cells [2] having 37.9% efficiency under 10× sunlight concentration have been reported. Moreover, referring to FIG. 2a, some tandem cell structures realized on Ge substrates (see FIG. 2a for materials) can potentially produce efficiencies of up to 63.1% [3,4].
Unfortunately, however, as can be seen from the solar spectrum illustrated in FIG. 2b, these multi junction III-V cells (i.e. cells that are at least partially constructed from Group III-V semiconductor materials) do not efficiently harness photons having energies above 2.2 eV. Generally, the cells described in References 1-5 are GaAs and/or Ge based multi-junction structures that may include GaInP, GaInAs and GaInNAs layers with tunnel junctions. For example, one factor that limits the attainment of a predicted efficiency in triple junction III-V cells is the adverse affect of band offset voltage based losses [5]. Table I immediately below shows the offset voltage in multi junction solar cells [5].
TABLE IOffset Voltage (based loss) in Multi-junction III-V Solar CellsSemiconductorBand gap (eV)Offset Volt [(Eg/e) − Voc]GaAs1.44 eV0.387 VGa0.83In0.17As1.118 eV 0.390 VGaInNAs1.07 eV0.600 VMoreover, using Ge and GaAs substrates is relatively expensive and these materials are not abundant in nature.
As such, a method for growing tandem cells is needed whereby Si substrates (single crystal and/or polycrystalline) and Si-on-glass or Si-on-quartz or Si-on-sapphire are used for the growth of Ge epitaxial layers (with reduced dislocation densities resulting from lattice mismatch), wherein the tandem cells efficiently harness photons having energies above 2.2 eV.